Beta-lactamase enzymes are a major mechanism of resistance among bacteria to the beta-lactam antibiotics, which include penicillins and cephalosporins. The beta-lactamases can be divided into three basic types. Classes A and C are serine enzymes and comprise the majority of the beta-lactamases identified to date (1,2). These enzymes generally inactivate penicillins or cephalosporins best and often show a preference for one of these two beta-lactam classes (1,2). The Class B beta-lactamases are unique in that they require a divalent cation, Zn.sup.2+, and that they are able to inactivate nearly every class of beta-lactam antibiotic (1). In addition, they are resistant to inactivation by beta-lactamase blocking agents such as clavulanic acid (3,4). Only a limited number of organisms are known to produce this type of enzyme. These include; Bacillus cereus (B. cereus), Xanthamonas maltophilia, Flavobacteruim odoratum, and Bacteroides fragilis (3,4,5,6). The relatedness of the enzymes from these organisms is unknown. The enzyme from B. cereus is the best characterized. Its DNA and protein sequence have been determined and some x-ray crystal structure is known (1,7,8). Three histidine residues and one cysteine residue have been identified as participating in binding the Zn.sup.2+ cofactor (9,10). The beta-lactamase gene from X. maltophilia has recently been cloned and sequenced (11,12).
The Class B beta-lactamase enzymes have already been identified (but not fully characterized) among clinical isolates of B. fragilis and pose a problem with X. maltophilia infections as the beta-lactamase gene is endogenous to X. maltophilia. In addition, Class B enzymes have been identified among other bacterial species. The presence of a Class B beta-lactamase precludes treatment of the infection with beta-lactam antibiotics. Thus, the Class B enzyme poses a problem in treating these infections and may become an increasing problem in the future as the beta-lactamase gene becomes more widespread.